1. Field of the Invention
The present invention relates to a thermistor and a method for producing the same. More particularly, it relates to a thermistor comprising a heat sensitive element consisting of thin film diamond which can measure high temperatures and a method for producing such thermistor.
2. Description of the Prior Art
A thermistor is widely used as a temperature measuring sensor in a variety of apparatuses and instruments. The thermistor has many advantages such that it has a larger temperature coefficient than a thermocouple, that it can be used in a voltage-current range in which a temperature is relatively easily measured, and that it does not require zero adjustment. As a heat sensitive element material for the thermistor, used are glass, Mn-Ni base oxides, SiC, BaTiO.sub.3 and the like.
The currently used thermistors are roughly divided into two kinds according to their characteristics. In one of them, the resistance change is proportional to temperature change, and in the other of them, the resistance abruptly changes at or around a certain specific temperature.
The former type thermistor finds many industrial applications for temperature control since it has larger resistance change against temperature change than other temperature measuring methods such as the thermocouple. The conventional thermistor can measure a temperature as high as 300.degree. C. when SiC is used as a heat sensitive element. However, it cannot measure a temperature higher than 300.degree. C. and it has been desired to provide a thermistor which can measure a temperature from room temperature to about 500.degree. C. or higher.
Diamond is not only hard but also thermally and chemically stable and does not corrode in a corrosive atmosphere up to 800.degree. C. Further, since it has the largest thermal conductivity (20 W/cm.K) among all materials and comparatively small specific heat, it has a high response rate and a wide measurable temperature range up to high temperature.
Although pure diamond is a good electrical insulant up to about 500.degree. C., when the diamond contains an impurity such as boron, it shows semiconducting property at room temperature.
Natural diamond rarely contains such semiconductive diamond, which is named as a "IIb" type, and it was proposed to produce a thermistor by using such impurity-doped natural diamond (cf. G. B. Rogers and F. A. Raal, Rev. Sci. Instrum., 31 (1960) 663).
However, since the natural occurring semiconductive diamond is very rare and has largely fluctuating characteristics, it cannot be practically and industrially used.
Nowadays, diamond can be artificially synthesized under ultra high pressure such as 40,000 atm. or higher. According to the synthesis technique of diamond, semiconductive diamond containing an impurity such as boron and aluminum can be synthesized and used in the production of the thermistor (cf. U.S. Pat. No. 3,435,399 and L. F. Vereshchagin et al, Sov. Phys. Semicond.).
The synthesized semiconductive diamond can measure a temperature up to 800.degree. C. with good linearity and reproducibly synthesized. However, since it is synthesized by means of an ultra high pressure generating apparatus, it is expansive. The diamond crystal is separated out from a metal solvent, it is difficult to homogeneously distribute the impurity throughout the diamond crystal. In addition, shapes of each synthesized diamond crystals are different and should be processed to form a suitable shape for the thermistor. Since the diamond is the hardest material in the world, its processing is difficult and expensive, which increases a production cost of the thermistor.